Additionally convenient laboratory test for harden-ability is Jominy test. A standard test piece of steel as illustrated in figure of The Jominy Test of Harden-ability is austenitized in usual manner. The lower end of the test piece is then quenched via a standard jet of water, resulting in a varying cooling rate.
Figure: The Jominy Test of Hardenability
The cooling rate at the jet end is about 300oC/sec whilst such at the other end is about 3oC/sec. it varying rate of cooling produces a broad range of hardness along the length of Jominy test piece. A flat portion is ground beside the length and hardness measured at different points. The plot of hardness along the length offers Jominy index of harden-ability.
The best utilization of the Jominy curve is made via drawing a horizontal line consequent to hardness of the semi-martensite zone. The semi-martensite zone hardness is explained in Table no.11. The point where such line requires the Jominy curve calculates the distance from the quenched end that can be inserted in figure of calculating the Diameter of Fully Hardened Articles as per to the Distance from the Quenched End to find out the diameter for a particular steel that will be fully hardened in water or oil.
Figure: Harden-ability Curves Plotted from End Quench Test Data
a) For Shallow; and
b) For Deep Hardening Steel
Figure: finding the Diameter of Fully Hardened Articles as per to the Distance from the Quenched End
Table no.1: Relationship between the Hardness of the Semi-Martensite Zone and the Carbon Content
Hardness of the Semi-martensite (Rc)
0.08 - 0.17
0.13 - 0.22
0.23 - 0.27
0.28 - 0.32
0.33 - 0.42
0.43 - 0.52
0.53 - 0.62